Design And Research On Heterodyne Planer Grating Encoder With Nanometer Resolution

Measurement technique is closely connected with the development of science and industrial technology. On the demand of the requirements of the more and more measurement accuracy in modern science and technology, especially the development in information technology, MEMS technology, aerospace technology, many new theories and methods are developed to improve the accuracy and efficiency of the measurement, and promote the modern technologies’ advance. With the rapid progress in the field of nanotechnology, there is high demand for new dis-placement measurement technologies to be able to achieve sub-nanometer level resolution with long-term reliability. These techniques are commonly applied to the measurement tools for machine tools, pick and place machines, coordinate measuring machines and semiconductor manufacturing equipment because the translational or rotational stages play an important role for determining the machine accuracy itself. In precise mechanics, electronics and MEMS’ industries, the manufacture technique requires precise measurement and location, especially in coordinate measuring machine (CMM) and computer numerical control (CNC) etc.In micro-displacement measurement, the grating measurement system is widely used for its merits of high accuracy, large range, stability, not so strict measurement environment. Especially with the rapid development of grating interferometer in recent years, the accuracy has achieved the ability of the sub-nanometer. A plane grating encoder is able to measure two-dimensional displacements simultaneously. Compared to a laser interferometer, a planar grating encoder is more stable to environment turbulence and less expensive. Compared with using two linear encoders to measure planar displacements, using a planar grating encoder can reduce the Abbe error caused by multiple measuring points and remove the non-orthogonality error caused by installation of two linear encoders. With the development of planar motors, planar grating encoders have many applications in the fields of precision machining, nano-measurement, etc. In practice, however, the optical interferometer used in non-isolation always suffers from large errors or noise, most commonly from atmospheric influences, background vibration, and thermal drift. The accuracy of the measurements in air strongly depends on the refractive index fluctuations. High-resolution optical encoders contain small grating interferometers with equal and short optical path lengths in both interferometer arms. Thus they are almost not affected by air turbulences and can show noise levels below 100 pm. By using proper materials and designs also the temporal and thermal drift can be reduced to sub-nanometer levels at appropriate environmental conditions.In this paper, the author researched the key technology of high precision plane grating interferometer in depth. The requirements of grating interferometer measurement.are analyzed in detail in view of the application of the wafer stage of lithography machine. Then summarized the related design method, including the interference information, effective use of diffraction light, optical multiplier and the mode select of signal processing, etc.In addition by Jones matrix method of modeling and numerical simulation are analyzed in detail that exist in the grating interferometer aliasing error and geometric error, uncertainty of measurement of grating interferometer is given.In the final design method and error analysis is designed under the guidance of a new type of high precision three degrees of freedom plane grating interferometer measurement system. The measurement system can simultaneously measure the direction of the grating vector and raster graphic method to the linear displacement. The grating vector direction can realize two degree of freedom in a wide range measurement, grating plane method to a range of plus or minus 1 mm measurement can be realized, in order to meet the lithography mechanic parts of six degrees of freedom displacement measurement. Adopts heterodyne signal processing method in the design and reduce the environment influence on displacement measurement, improve signal-to-noise ratio, heterodyne signal processing and segmentation, at the same time, simplify the optical structure of the interferometer. With optical and electronic segment, segment can realize three degrees of freedom of nanometer resolution displacement measurements. In addition, the short optical path can ensure the measurement accuracy is less affected from outside environment. What is more, this design has small volum and light quality, which can enhance displacement measurement precision and dynamic performance of stage.